This invention relates to dynamoelectric machines. More particularly, it relates to plastic end shields for use in such dynamoelectric machines as motors, generators, and the like.
Conventional dynamoelectric machines typically include a housing, a rotor assembly rotatably disposed in the housing, and one or more end shields which provide structural support for associated axial ends of the rotor assembly. The end shields are usually made of steel, aluminum, cast iron, or other die cast alloy. Plastic end shields have also been used in the past, but only for very small motors. Plastic end shields have typically been employed in motors for such applications as toys and small appliances, where very little heat is generated inside the motor housing. For such applications, employing plastic end shields as a substitute for metal end shields has required little more than using a plastic material in the same mold that would be used to make a metal end shield for the same application.
However, for more demanding applications, such as, for example, motors of horsepower ratings above 0.05 horsepower, a significant amount of heat is generated inside the motor housing, and the end shields for the motor are subjected to temperatures as high as 200.degree. C. While such temperatures do not present any significant problems when metal end shields are employed, such elevated temperatures in plastic end shields may cause deterioration of the mechanical properties of the plastic material, and excessive creep in the plastic structure. Most of the relatively inexpensive plastic materials currently available exhibit durable mechanical properties at relatively low temperatures. At higher temperatures, however, the mechanical properties of the material deteriorate significantly. As a result, end shields made of such materials may fail in their primary function of maintaining the proper distance between the outermost radius of the rotor assembly and the innermost radius of the housing, which distance is often referred to as the rotor-stator air gap. Plastic materials capable of withstanding higher temperatures are available, but such materials are relatively expensive and generally do not offer any cost savings over metal end shields. Thus, while end shields made from inexpensive plastic materials would be much lighter than the metal end shields currently employed, and would offer the potential for reducing manufacturing costs, it is not feasible to directly substitute such plastic materials in the metal end shield designs currently employed for dynamoelectric machines which generate a significant amount of heat inside the housing.
Furthermore, the primary motivation for replacing metal end shields with plastic end shields is not the cost of the material itself (which, in many applications, is greater for plastics than for metals), but rather a reduction in the overall manufacturing cost of the end shield. Because relatively complex parts can be molded from plastics, a metal assembly involving several different components can often be replaced by a single plastic part, at a lower overall manufacturing cost. Thus, an effective exploitation of the advantages provided by using plastics to form an end shield requires a design in which multi-functionality can be incorporated into the end shield. In dynamoelectric machines, besides serving to maintain the rotor-stator air gap, the end shields also often perform such functions as providing an oil reservoir or grease cavity around the bearings, serving as a mounting bracket, or providing a mount for such accessories as tachometers, brakes, blowers, switches, or brush rings. Plastic end shields can be designed to combine several of these functions. For example, since plastic is an electrically insulative material, the brush support ring used in D.C. machines can be formed as an integral part of the end shield. By providing multi-functionality in this manner, plastic end shields can offer significant reductions in overall manufacturing costs.
Accordingly, it is an object of the present invention to provide an end shield for dynamoelectric machines which is light in weight and inexpensive to manufacture.
It is also an object of the present invention to provide an end shield capable of withstanding relatively high temperatures caused by heat generated inside the machine housing.
It is a further object of the present invention to provide an end shield which is made from plastic.